Current transmission tendencies are to utilize ever higher transmission rates. However, these rates are not always ensured, for the transmission mediums are subjected to disturbance degrading this transmission and often lower rates which are less sensitive to disturbance have to be selected.
Therefore, the problem occurs of adapting the transmission rate between two modems (a local modem and a remote modem) as a function of the quality of the line and throughput capacity of the modems. This is described in European Patent Application No. 0 643 507 filed Sep. 6, 1994. Although the known system gives entire satisfaction, it is no longer adapted to the new standards that are applied to the modems; one may recollect specifically V.34 standard of CCITT which proposes for certain rates multiple combinations of carrier frequencies and symbol rates.